1. Technical Field of the Invention
The present invention relates to a steering apparatus for a vehicle that directs steerable wheels in response to operator input in which the steerable wheels are not mechanically coupled to the manually steerable member.
2. Description of the Prior Art
A typical automotive vehicle is steered by transmitting operations of a manually steerable member, such as a steering wheel, to a steering mechanism for directing steerable wheels for steering. Generally, the manually steerable member is located inside the vehicle passenger compartment, and the steerable wheels are located at the front of the vehicle. Thus, a suitable steering mechanism is necessary to couple the manually steerable member and the steerable wheels.
A representative steering mechanism is a rack-and-pinion type steering mechanism. In a rack-and-pinion steering mechanism, the rotational motion of the steering wheel is communicated through a steering column to a pinion gear at its distal end. The pinion gear is engaged with a rack gear disposed laterally between the steerable wheels, which in turn are coupled to the rack gear by knuckle arms and tie rods. In this manner, rotation of the steering wheel is translated into the lateral movement of the rack gear, which causes the steerable wheels to pivot in the desired direction. In general, mechanical steering mechanisms are power-assisted by hydraulic or electrical assist units.
Mechanical steering mechanisms such as described above have a number of limitations. As the manually steerable member and the steering mechanism are mechanically coupled in some fashion, the position of the manually steerable member is limited within the vehicle passenger compartment. Moreover, the size and weight of the coupling members limits the layout and performance of the vehicle. Lastly, representative steering mechanisms are designed for the receipt of a rotational input from the operator, meaning that the manually steerable member is generally a steering wheel. Thus, alternative steerable members, such as levers, handgrips, and pedals have been used in only limited circumstances.
In order to overcome such limitations, it has been proposed to utilize a steering system in which the manually steerable member is not mechanically coupled to the steerable wheels and the steerable wheels and steering movement is achieved by an electrically controlled motor, a so-called steer-by-wire system. In a steer-by-wire system, a steering actuator operates in response to detected values of various steering parameters, such as steering wheel angle and vehicle speed etc. The detected values are communicated electronically to the steering actuator from sensors, whereby the steering actuator orients the steerable wheels in the desired direction.
Steer-by-wire systems solve a number of problems presented above. In addition, there are a number of other advantages innate to steer-by-wire systems that were not apparent in its mechanically coupled counterpart. A steer-by-wire steering system can be easily integrated into other electronically controlled systems to increase the efficiency and performance of the vehicle.
Although a steer-by-wire system does present the foregoing advantages, it also presents a number of problems. Since there is no direct mechanical coupling between the operator and the steerable wheels, the operator does not receive any feedback from the road surface through the steering mechanism. In order to solve this problem, engineers have employed a reaction torque motor to simulate the feedback experienced by the vehicle operator. The reaction torque motor generates a reaction torque, generally to a steering wheel, based upon a number of steering parameters such as vehicle speed, steering wheel angle, and road surface condition.
U.S. Pat. No. 6,079,513 discloses a steering apparatus for a vehicle comprising a calculating means for calculating a target value for the reaction torque. The target value is characterized by a self-aligning torque term, an elastic resistance term, and an inertial resistance term. The self-aligning torque term is based on the detected values of vehicle speed and position of the steering mechanism. The elastic resistance term is proportional to the deviation between a detected value for a steering operating angle of a steering operating means and the detected value for the actual position. The inertial resistance term is proportional to a time-varying amount of the calculated deviation. Further disclosed was a turning condition sensor and means for correcting the reaction torque applied to the steerable member in the event that the vehicle approached its cornering limit in accordance with detected values of the vehicles turning condition.
The target value approach to generating a reaction torque in a steer-by-wire program does, however, present several problems. For example, the target value approach does not accurately reflect the instantaneous condition of the steerable wheels because the reaction torque is not calculated arithmetically based upon the currently sensed steering conditions. Additionally, the target values must be calibrated for varying driving conditions, involving a tedious process and increasing the amount of labor expended by engineers to tune the system.
The present invention has been devised in order to solve the above problems. The present invention provides a steer-by-wire steering system capable of transmitting a reaction torque to a manually steerable member that is arithmetically computed and speed sensitive. The present invention generates the reaction torque by monitoring and sensing a plurality of driving conditions and then adapting the reaction torque applied to the manually steerable member based upon vehicle speed.
The principal features of this invention include a plurality of driving condition sensors or estimators adapted for determining (1) steering actuator load, (2) wheel slip angle, (3) steering angle, (4) yaw rate, (5) lateral acceleration and (6) vehicle speed. The sensors electronically transmit input values to a control unit that is adapted for arithmetically calculating initial reaction torque values in response thereto, assembling the reaction torque values into a set of blended reaction torque values, normalizing the blended reaction torque values into a final reaction torque value, and transmitting the final reaction torque value to a feedback generator.
Initially, the control unit selects a set of tuning parameters for each input signal. The tuning parameters are defined as: Api, Ahi, Bpi, Bhi, and their physical significance is discussed further herein. Each sensor produces an input signal Fi that is independently operated upon by a plurality of reaction torque equations, SWTp(Fi) and SWTh(Fi), for arithmetically computing a set of initial reaction torque values. The control unit then blends the initial reaction torque values based upon the vehicle speed, v, and a speed blending parameter, kbi(V), thereby producing a set of blended reaction torque values, SWT(Fi). Each blended reaction torque value is then assigned a weighting constant, wi, where wi is determined by the vehicle speed. To calculate the final reaction torque, T(Fi,v), the control unit sums the product SWT(Fi)wi, and transmits this value to the feedback generator whereby reaction torque is generated in the manually steerable member.
Thus, the apparatus of the present invention obtains empirical data from a plurality of sensors and processes that data arithmetically in real time. The present invention further processes the empirical data by assigning a weighting factor to the reaction torque calculated from each input signal, as a function of vehicle speed. For example, at low vehicle speeds, yaw rate, lateral acceleration, actuator load and wheel slip angle are poor indicators of the feedback generally transmitted to a driver by a mechanically coupled steering mechanism. Consequently, each of the reaction torque values calculated in response to these input values is assigned a relatively low weighting factor. Conversely, at low vehicle speeds, steering angle is an important indicator of the feedback generally transmitted to a driver. Therefore, reaction torque value calculated in response to this is assigned a relatively high weighting factor.
By way of comparison, at high vehicle speeds actuator load, yaw rate, lateral acceleration, and wheel slip angle are good indicators of the feedback generally transmitted to a driver by a mechanically coupled steering mechanism. Consequently, each of the reaction torque values calculated in response to these input values are assigned a relatively high weighting factor.
Thus, in the present invention the reaction torque values calculated from each input signals are appropriately weighted as a function of vehicle speed and the steering feel desired. After the weighting process is completed, the initial reaction torque values are summed together to determine a final reaction torque value that is transmitted to the feedback generator whereby feedback is generated in the manually steerable member.
The present invention further encompasses an alternative steering system in which the steerable wheels have independent steering actuators rather than a unitary linkage disposed laterally between the steerable wheels. In this embodiment, there is at least one steering actuator load sensor disposed at the independent steering actuators for measuring the load placed on the actuators by the steerable wheels.